THE STRUCTURE OF THE BRAIN STEM 



369 



opposite side or end in connection with the nerve cells which are scattered 

 throughout the white fibres. The pyramids can still be seen as thick longi- 

 tudinal bundles on each side in the midst of the transverse fibres. They are 

 considerably larger than in the medulla and become larger as we trace them 

 up towards the mid-brain, owing to the presence of a number of fibies 

 which are derived from the cortex cerebri and end in the grey matter of the 



Supr. cer. peduncle 



Rt. of 5th n- 



Valve of Vieussens 



Motor nucleus of 5th n 



Sensory nucleus of 5th n 

 Supr. olive 



snsory root 

 of 5th n. 



Idle peduncli 

 of cerebellun 



Form reticularis 



Corpus 

 trapezoides 



Pyramids 



FIG. 185. Transverse section through middle Of pons Varolii of orang on level of 

 nuclei of fifth nerve. (CUNNINGHAM.) 



pons. The tract of the fillet lies on each side of the middle line dorsally to 

 the transverse fibres. A little to the outside of the fillet is seen a special 

 mass of grey matter, known as the superior olive. The nervous mass lying 

 behind the transverse fibres of the pons, between them and the grey matter 

 of the floor of the fourth ventricle, is known as the formatio reticularis. It 

 is divided into a lateral and mesial part by the fibres of the hypoglossal 

 nerve. In the lateral portions there is a considerable quantity of grey 

 matter, which can be regarded as continuous with the grey matter of the 

 lateral horns of the cord. The c lateral nucleus ' is simply a condensed part 

 of this grey matter, lying between the olive and the gelatinous substance of 

 Rolando. The mesial part of the formatio reticularis is almost free of nerve 

 cells. The reticular appearance of this part of the pons is due to the inter- 



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